1. Field
The disclosed concept pertains generally to electrical distribution panels and, more particularly, to electrical distribution panels being configured or being configurable to accommodate non-critical loads and critical loads supplied with power from a second power source in response to power from a first power source becoming unacceptable.
2. Backgound Information
Electrical distribution panels, such as load centers, incorporate a plurality of circuit breakers and provide a safe and controllable distribution of electric power. Such load centers have become a common feature in both residential and commercial applications. Increasingly, such load centers are utilized in installations that incorporate, for example, an electric generator as a second power source in the event that a utility service serving as a first power source fails or becomes unacceptable.
With technological progress resulting in ever more uses for electricity, the amount of electrical power required for both residential and commercial applications has steadily increased, and this had lead to increased demand for relatively larger electric generators. Unfortunately, relatively larger electric generators present various disadvantages over relatively smaller electric generators. While smaller electric generators are typically air cooled, larger electric generators typically require a liquid cooling system with a circulation pump and radiator, thereby adding to both the costs and complexities of operating and maintaining a larger generator in comparison to a smaller generator. Larger generators also require relatively larger quantities of maintenance fluids, including lubrication oil and coolant liquid.
A known proposal for either new construction or the retrofit of existing installations involves the addition of a separate load center panel for critical circuits. This separate load center panel receives a backup power source and, also, manually switches between a utility power source and the backup power source. This can be utilized in installations that incorporate the backup power source, such as an electric generator, in the event that the utility power source becomes unacceptable (e.g., without limitation, fails; becomes unreliable; becomes unavailable). This can provide, for instance, reliable electric power for doctor's offices away from hospitals, home-based businesses and home-based chronic patient care support. For existing installations, this requires that the critical circuits be moved (e.g., rewired) from a first load center to the separate load center panel. However, it requires significant time and effort to rewire a load center in order to electrically connect a backup power source, such as an electric generator or other auxiliary power unit (APU) (e.g., a device whose purpose is to provide electrical energy), to critical circuits in, for example, residential and relatively smaller scale commercial structures.
Transfer switches are well known in the art. See, for example, U.S. Pat. Nos. 6,181,028; 5,397,868; 5,210,685; 4,894,796; and 4,747,061. Transfer switches operate, for example, to transfer a power consuming load from a circuit with a normal power supply to a circuit with an auxiliary power supply. Applications for transfer switches include stand-by applications, among others, in which the auxiliary power supply stands-by if the normal power supply should fail. Facilities having a critical requirement for continuous electric power, such as hospitals, certain plant processes, computer installations, and the like, have a standby power source, often a diesel generator. A transfer switch controls electrical connection of the utility lines and the generator to the facility load buses. In many installations, the transfer switch automatically starts the generator and electrically connects it to the load bus upon loss of utility power, and electrically reconnects the utility power source to the load bus if utility power is reestablished.
Another known proposal employs a single interlock between a main circuit breaker and a manual transfer switch. In response to loss of utility power, the user must first manually turn off any non-critical circuits, turn off the main circuit breaker, and then turn on the transfer switch. The manual sequence is reversed when utility power has returned.
Other known proposals provide mechanical interlocks between a main circuit breaker and a generator circuit breaker.
Further known proposals require that the entire load be switched from the utility power source to the generator power source. In other words, the loads are not separated into critical loads and non-critical loads.
U.S. patent application Ser. No. 12/043,514 discloses a first bus powered from a first circuit breaker and a first power input, a second bus, an automatic transfer switch including a first input electrically connected to the first bus, a second input electrically connected to a second power input, and an output electrically connected to the second bus, and a number of pairs of circuit breakers. The number of pairs of circuit breakers include a second circuit breaker powered from the first bus, a third circuit breaker powered from the second bus, a power output powered from the second and third circuit breakers, and an interlock cooperating with the second and third circuit breakers and structured to prevent both of the second and third circuit breakers from being closed at the same time.
It is known to provide a load center that can accept an automatic transfer switch.
NEC 2008, Optional Standby Systems, provides in Section 702.5(2)(a)-(b) that where automatic transfer equipment is used, an optional standby system shall comply with either: (a) the standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment, or (b) where a system is employed that will automatically manage the electrically connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be electrically connected by the load management system. Hence, for an automatic transfer switch, the standby system must be able to handle the entire load that is transferred.
A suitable manual switch/interlock arrangement is used with a portable generator. This is because Article 702 of the NEC requires that a load center or panelboard cannot be simultaneously energized from two different power sources. Hence, the user must turn off a utility source input before turning on a generator source input. For example, a mechanical interlock prevents accidently backfeeding onto the utility source input since this could, otherwise, cause equipment failure, fire, or possible death due unexpected energized utility power lines. Also, portable generators are not setup to automatically start up and transfer power, which must be done manually.
In a “separately derived” system, the neutral and ground are electrically bonded together by a system bonding jumper at the generator. This neutral and ground are typically protected by a ground fault circuit interrupter (GFCI) at the generator. Failure to install the system bonding jumper correctly can result in nuisance tripping of this GFCI.
In a “non-separately derived” system, the neutral and ground are not electrically bonded at the generator. Instead, the neutral assemblies for the utility source and the generator source are electrically connected together at the load center. Failure to properly make that electrical connection could lead to equipment failure, fire, or possible death due to a floating neutral condition.
U.S. patent application Ser. No. 12/172,504 discloses an electrical distribution panel including an enclosure comprising a first compartment and a separate second compartment, a first power input, a first circuit interrupter including a first terminal electrically connected to the first power input and a second terminal, a first bus electrically connected to the second terminal of the first circuit interrupter, a plurality of second circuit interrupters powered from the first bus, a second bus electrically connected to the first bus through one of the second circuit interrupters, and a number of third circuit interrupters powered from the second bus. The first bus and the number of second circuit interrupters are structured to power only a number of first loads. The second bus and the number of third circuit interrupters are structured to power only a number of second loads. The second compartment is structured to receive an automatic transfer switch including a first input electrically connectable to the first bus, a second input electrically connectable to a second power input, and an output electrically connectable to the second bus. The automatic transfer switch is structured to selectively electrically connect one of the first and second inputs of the automatic transfer switch to the output of the automatic transfer switch. This enables a user, such as a home owner, to install an electrical distribution panel, such as, a load center panel, at the time of construction and use that load center panel as a conventional load center until, at a later date, they can afford to purchase and install a second power source (e.g., without limitation, a generator) and an automatic transfer switch. Hence, the electrical distribution panel provides a load center that is ready to receive an automatic transfer switch and operate with an automatic, standby generator of a non-separately derived power system. An automatic transfer switch kit includes an interior assembly having an automatic transfer switch and a corresponding wire harness.
Referring to FIG. 1, a load center 2 includes an enclosure assembly 4, a trim assembly (not shown), a first interior assembly 8 and a second interior assembly 10. As is conventional, the load center 2 also includes a ground bar assembly 12 and a number of neutral bar assemblies 14. The load center 2 is divided into a first or upper (with respect to FIG. 1) section 16 containing the first interior assembly 8 and a second or lower (with respect to FIG. 1) section 18 containing the second interior assembly 10. A barrier 20 preferably separates the first section 16 from the second section 18. For example, the barrier 20 physically separates the sections 16,18 for UL purposes. The section 18 preferably meets panelboard standards under UL 67, and transfer switch standards under UL 1008. A wire harness 22 electrically connects the first interior assembly 8 and the second interior assembly 10 as will be described.
As is conventional, the load center 2 includes a main circuit breaker 24 (e.g., without limitation, two poles, 200 A). The main circuit breaker 24 provides power from a first power input 25 for a first power source (e.g., without limitation, utility; primary) 26 (shown in phantom line drawing) to a first bus 28 (e.g., without limitation, 120 VAC and/or 240 VAC) of the first interior assembly 8, which first bus 28 includes a number of circuit breakers of which only example circuit breaker 30 (e.g., without limitation, two pole, 50 A) and circuit breaker 31 (shown in phantom line drawing) are shown. Although two-pole circuit breakers are shown, the load center 2 can include circuit interrupters having any suitable number of poles. The main circuit breaker 24 includes a number of first or line terminals 27 electrically connected to the first power input 25 and a number of second or load terminals 29. The first bus 28 is electrically connected to the number of second or load terminals 29 of the main circuit breaker 24. The circuit breaker 30 includes a number of line terminals 41 (e.g., without limitation, two line terminals are shown) electrically connected to the first bus 28.
The first interior assembly 8 also includes a separate second bus 32 (e.g., without limitation, 120 VAC and/or 240 VAC), which second bus 32 includes a number of circuit breakers of which only circuit breakers 33,34 (shown in phantom line drawing) are shown. As will be explained, only circuit breakers, such as 33,34, of the separate second bus 32 are employed to power critical loads. Also, only circuit breakers, such as 31, of the first bus 28 are employed to power non-critical loads. The circuit breakers 30,31 operate independently from (e.g., without limitation, do not require any interlock therebetween) the circuit breakers 33,34.
The wire harness 22 electrically connects the load terminals 39 of the circuit breaker 30 to first input terminals 36 of an automatic transfer switch (ATS) 38 of the second interior assembly 10. The wire harness 22 also electrically connects the input terminals 40 of a sub-feed lug block 42 to the output terminals 44 of the ATS 38. The sub-feed lug block 42, thus, electrically connects the output terminals 44 of the ATS 38 to the second bus 32.
As is conventional, the ATS 38 includes second input terminals 46 for receiving power from a second power input 47 for a second power source 48 (e.g., without limitation, backup; a generator; an auxiliary power unit; an uninterruptible power source).
During normal operation, the critical circuits powered from the second bus 32 are energized by the ATS 38 with power from the first power source 26. As is conventional, the non-critical circuits powered from the first bus 28 are always energized by power, if available, from the first power source 26. When the power from the first power source 26 is interrupted, the ATS 38 recognizes that loss of power, automatically starts, for example, the generator 48, and energizes only the critical circuits powered from the second bus 32 with power from the generator 48 through the output terminals 44 of the ATS 38 and through the sub-feed lug block 42.
Referring to FIG. 2, another load center 50 is shown. The load center 50 is similar to the load center 2 of FIG. 1, except that the second interior assembly 10 including the ATS 38 and the wire harness 22 are not included. Instead, in FIG. 2, the branch mounted circuit breaker 30 (e.g., without limitation, CH250 marketed by Eaton Electrical, Inc. of Pittsburgh, Pa.) is electrically connected by jumper assembly 52 to the sub-feed lug block 42 (e.g., without limitation, CHSF2125 marketed by Eaton Electrical, Inc. of Pittsburgh, Pa.) to energize, when the circuit breaker 30 is closed, a number of critical circuits powered from the second bus 32 of the “split bus” (i.e., first bus 28 is electrically split apart from second bus 32) interior. Other non-critical circuits are terminated as part of the first bus 28 of the “split bus” interior. The jumper assembly 52 electrically connects the load terminals 39 of the circuit breaker 30 to the input terminals 40 of the sub-feed lug block 42. The jumper assembly 52 includes jumpers or wires (conductors) that supply power from the branch mounted circuit breaker 30 to the sub-feed lug block 42. For example, these jumpers or wires electrically connect the A and B legs of the first bus 28 (see, for example, the load terminals 39 of the circuit breaker 30) to the corresponding A and B legs (see, for example, the input terminals 40 of the sub-feed lug block 42) of the second bus 32.
There is room for improvement in electrical distribution panels.